Blown fibre installations may comprise ducting or any type of passage along which an optical fibre cable, or an optical fibre unit can be blown. Optical fibre units suitable for blown installation are known, for example from U.S. Pat. No. 5,533,164 and U.S. Pat. No. 5,042,907. Such optical fibre units may include single fibres, bundles of fibres or ribbon of fibres. Optical fibre cables suitable for blown installation are known, for example, from US 2003/0228116A1 and U.S. Pat. No. 4,976,519.
Optical fibre units or cables can be blown, for example, inside cables previously installed, as described for example in U.S. Pat. No. 6,101,304. US 2004/0037522 and US 2003/0123824 or into ducts properly designed, such as described in U.S. Pat. No. 6,328,283 and U.S. Pat. No. 6,311,267.
Methods for installing an optical cable into a duct are known, for example, from U.S. Pat. No. 6,311,953 and U.S. Pat. No. 5,645,267.
Ducting for blown installation of optical fibre units or optical fibre cables can be up to 5 km long and typical internal diameters for such ducting are 20, 25, 33 and 40 mm. The optical fibre cables that are blown along such ducts will typically have a diameter of 13 to 25 mm. Such ducting systems may be installed long before the optical fibre cable is installed and will often be installed by someone other than the installer of the cable. As a consequence, the person installing the optical fibre cable may not know the length of the ducting.
It is important to know the length of the ducting so that the maximum installation distance of the optical fibre cable is not exceeded. It may be possible to measure the ducting length using a measuring wheel. However, this will be a time-consuming process in view of the typical installation lengths and in many cases, may not be possible. This is because it is often not possible to gain access to the route taken by the ducting, which will typically be buried, or otherwise hidden from view.
It is also important for the installer of the optical fibre cable to know the condition of the ducting. Ducting systems are typically made up of a plurality of lengths of ducting connected together using air-tight connectors. If the connectors are not assembled correctly, it is possible for there to be an air leak at the duct/connection interface. Such air leaks reduce the airflow through the ducting system and reduce the installation distance for the optical fibre cable.